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Ice Dams

Q: Last winter our home had extensive water damage that was caused by ice dams. What is the main cause of ice dams and is there an easy way to prevent them?

A: The formation of an ice dam requires three basic ingredients:

  1. Sub-zero temperatures
  2. Snow covering the roof
  3. Non-uniform heating of the roof surface

This complex interaction can occur when the roof has an accumulation of snow and an area of the roof becomes warm enough to melt the snow. The meltwater will flow under this blanket of snow until it reaches a section of the roof that is cold enough to refreeze. The more repetitive the melting and freezing cycle, the greater the chance that an ice ridge or ice dam will build up to sufficient height that it could force water to back-up under the roof shingles and this will eventually lead to water leakage into the attic space or exterior wall cavities. 

Even on sub-zero winter days, the sun emits enough solar radiation that it can warm up any dark surface to above the temperature that snow melts. Anyone with an asphalt driveway appreciates how it attracts and holds more of the sun’s heat and this allows the snow to melt faster. When it comes to roof surfaces, any section of roof exposed to solar radiation will typically melt first, but as the meltwater flows down the roof it will reach a lower surface that is still covered in snow and will gradually refreeze. This refreezing normally happens over the soffit area, which is the lower edge of the roof near the gutters. It is also possible that unique roof designs involving hips, valleys, dormers or a roof that has changes in slope, as well as some types of mechanical system penetrations, such as plumbing stacks, chimneys, ceiling pot lights, bathroom exhaust fans, etc. could increase the risk for ice dams.  

However, it has been my experience that the primary reason for snow to melt is simply from building heat loss. If any heat from inside the home can make it into the attic space, the snow on the roof will melt. That is why it is so important that every attic be properly sealed, ventilated and insulated with at least 10 to 12 inches (R40) of blown-in insulation. More insulation will always be better, but any gaps or penetrations in the building envelope air barrier or irregularities in the insulation thickness can cause heat to escape. It is also equally important to ensure the ventilation in the attic space is sufficient to keep the roof surface cold. If you’re able to keep the temperature in the attic the same as the outside air, then the probability of getting an ice dam is significantly reduced. 

A house is a complicated system of various interactive components and each home will have its own set of variables as to what is causing the ice dams, as well as what possible unique solution might be required to resolve it. If your house has ice dams, I would first recommend you consult with a professional roofer who should have the proper equipment and training to assist you. 

Lawrence Englehart (Global Property Inspections) is a Registered Home Inspector and be reached at or or